So here's my problem: my wife is one of those "extremely rare" women you describe that has exclusively vaginal orgasms. She has never had one outside of penetrative intercourse. As such, she's not really interested in receiving oral sex. Problem is -- I love it! I wish she would let me go down on her more often, but it just doesn't do much for her. She's not too uncomfortable with my face planted between her legs to enjoy it. How can I help her get more comfortable with being on the receiving end (she gives great head, btw)??
Have you told her how much you enjoy doing oral sex? Have you made it clear that she doesn't have to come from it? Perhaps offering to do it right after both of you have showered would eliminate some of her concern. So many women fear their genitals are ugly and smelly. Any time you compliment her on how beautiful her vulva is might also help. Take her to our Genital Art Gallery and together look at all the different sex organs.
Finally after menopause when most vaginal linings thin out and vaginal penetration no longer feels that great, she might welcome oral sex then. One husband had tried to get his wife to let him go down on her for years. She would permit it occasionally while obviously not enjoying it. But in her mid-fifties after menopause, she welcomed his talented wet tongue. It's a shame but you might have to wait her out.